First let me say that this whole engine swap has proven to me that as usual there is more than one way to skin a cat, just as long as you're happy and truck is runnig good that's all that matters at the end.
Original 3.4 manifolds were almost new looking where as my old 2.8s were all shot to hell, plus I broke off half the studs trying to get them out. So using ones out of 3.4 was more of a necessity. I had no problem fitting them to the rest of my exhaust system, some time before engine change I put all new 2" pipes all the way back, even with the old 2.8 I noticed a difference right away in engine performance. It has a deeper lower pitch sound to it vs. the original stock. My main issue with the 02 sensor is which one to use, the original because its so far back on the pipe takes quite a bit longer to warm up and as a result it keeps ECM on by-pass loop longer. New sensors are installed right in the manifold it self and have a much better cold start performance. Only issue is where they are placed for oxygen content sensing in relation to Air pump. The original placment of 02 sensor takes into account all of the engines' exhaust gases, where as unit installed in the manifold will take each bank of cylinders as that of a whole engine +/- air pump depending where the sensor is.
I guess at the end I'm going to try get the set-up that yields best fuel economy. Having said that I'm wandering what kind of fuel economy are you guys getting once the engine swap is done and everything is working right?